5. Standards

5.1 GENERIC PROCEDURES

There are two primary procedures allowed by DoD policy for clearing
and purging AIS memory and secondary storage media that have processed
sensitive information: overwriting and degaussing. [1] Other procedures
are media specific and this section details them where appropriate. The
need for destruction arises when the media reaches the end of its useful
life.

5.1.1 OVERWRITING

Overwriting is a process whereby unclassified data are written to
storage locations that previously held sensitive data. To satisfy the
DoD clearing requirement, it is sufficient to write any character to all
data locations in question. To purge the AIS storage media, the DoD
requires overwriting with a pattern, then its complement, and finally
with another pattern; e.g., overwrite first with 0011 0101, followed by
11001010, then 1001 0111. The number of times an overwrite must be
accomplished depends on the storage media, sometimes on its sensitivity,
and sometimes on differing DoD component requirements. In any case, a
purge is not complete until a final overwrite is made using unclassified
data.

5.1.2 DEGAUSSING

Degaussing is a process whereby the magnetic media is erased, i.e.,
returned to its initial virgin state. To satisfy the DoD requirement on
degaussing a classified magnetic tape, the degausser must have met DoD
testing requirements as discussed in Section 3, "Degaussers."

5.1.3 DESTRUCTION

It is good practice to purge media before submitting it for
destruction. Media may generally be destroyed by one of the following
methods. (Although approved methods, options d and e use acid, which is
dangerous and excessive, to remove recording surfaces. Options a, b,
and c are recommended over d and e.)

5.2 SPECIFIC PROCEDURES

DoD 5200.28-M provides accepted DoD procedures to clear, purge,
declassify, and destroy storage media. This section, "Standards," is a
reflection of those procedures but does not provide the entire procedure
(e.g., use three overwrites to purge disks). This is because these
standards are evolving and this document, A Guide to Understanding Data
Remanence in Automated Information Systems, is not to be construed as
replacing policy.

5.2.1 MAGNETIC TAPES

Although overwriting can be used for clearing this media, the method
is time consuming and generally never used. Also, inter-record gaps may
preclude proper clearing. A better method for clearing Type 1,11, and
Ill tapes is degaussing with a Type l or Type II degausser. This
procedure is considered acceptable for clearing, but not purging, all
types of tapes.

Degaussing with an appropriate degausser is the only method the DoD
accepts for purging this media. Specifically, a Type I degausser can
purge only Type tapes, and Type II degaussers can purge Types l and Il
tapes. No degausser presently exists that is capable of purging Type
III tapes in accordance with NSA/CSS Specification L1 4-4-A.

5.2.2 MAGNETIC HARD DISKS

The DoD has approved both overwriting and degaussing as methods to
clear or purge this media. See Section 4, "Risk Considerations," and
DoD 5200.28-M for additional information. Degaussed disks will
generally require restoration of factory installed timing tracks. Type
I degaussers and approved hand-held magnets can purge this media up to a
coercivity level of 1100 oersteds. If hand-held magnets are used, then
the magnet must be placed in almost direct contact with the disk,
separated by only a tissue to prevent scratching the disk. Sometimes it
is possible to insert the magnet between the platters without
disassembling them. As a practical matter, if the drive must be
disassembled, it is usually easier to destroy the platters than to
degauss and then reinstall them.

Recently completed research has indicated that degaussing is an
effective method to purge rigid disk media. Large cavity degaussing
equipment can be used to erase the data from sealed disk packs and
Winchester style hard disk drives while the platters remain in the
drive. Care must be exercised to ensure that the disk drive is not
encasqd in a material that conducts a magnetic field. Research has
shown that aluminum housings on Winchester disk drives attenuate the
degaussing field by only about 2 db. Operational guidance is now being
developed for the DoD.

5.2.3 MAGNETIC DRUMS

The DoD has approved both overwriting and degaussing as methods to
clear or purge this media. See Section 4, "Risk Considerations," and
DoD 5200.28-M for additional information. Type l degaussers and
approved hand-held magnets can purge this media, with the latter being
the only practical alternative.

5.2.4 MAGNETIC FLOPPY DISKS AND CARDS

The DoD has approved overwriting for clearing, but not purging,
magnetic floppy disks. Degaussing is the preferred method. The
technology of magnetic cards is old and not generally used. Degaussing
with Type I degaussers or approved hand- held magnets is the only DoD
accepted method of purging floppy disks and cards, regardless of their
coercivity. See DoD 5200.28-M for additional information.

5.2.5 MAGNETIC CORE MEMORY

The DoD has approved both overwriting and degaussing as methods to
clear or purge magnetic core memory. Type l degaussers and hand-held
magnets can purge this media. See DoD 5200.28-M for additional
information.

5.2.6 PLATED WIRE MEMORY

There are restrictions on overwriting magnetic plated wire memory
based on the amount of time that information was resident in the same
memory location. See DoD 5200.28-M for additional information.

5.2.7 THIN FILM MEMORY

The DoD has approved both overwriting and degaussing as methods to
clear or purge thin film memory. Type l degaussers and approved
hand-held magnets can purge this media.

5.2.8 MAGNETIC BUBBLE MEMORY

The DoD has approved both overwriting and degaussing as methods to
clear or purge magnetic bubble memory. An alternative procedure for
magnetic bubble memory modules that have been designed with a built-in
bias voltage control is to adjust (i.e., raise) the bias voltage to a
level that would cause ttr;e collapse of all the magnetic bubbles. On
some bubble devices a chip erase is invoked by pulsing the z-coil. If
the memory was designed with a bias control, information will be
available from the vendor on the correct bias voltage level to apply to
cause the collapse of all the magnetic bubbles. Type l degaussers and
approved hand-held magnets can purge this media. Degaussed bubble
memory will generally require reinitialization with programs available
from the manufacturer. Bubble memory has not been shown to exhibit any
magnetic remanence after application of any of these purging methods.

5.2.9 RANDOM ACCESS MEMORY (RAM)

The DoD has approved both overwriting and removal of power as
methods to clear or purge RAM. See DoD 5200.28-M for additional
information.

5.2.10 READ ONLY MEMORY (ROM)

Because data is permanently stored in ROM, clearing and purging this
media has no relevance. See DoD 5200.28-M for additional information.

5.2.11 ERASABLE PROGRAMMABLE READ ONLY MEMORY

(UVPROM)

The DoD has approved the use of ultraviolet light to clear or purge UVPROM.
See DoD 5200.28-M for additional information.

5.2.12 ELECTRICALLY ERASABLE READ ONLY MEMORY (EEPROM)

The DoD has approved different forms of overwriting (e.g.,
single-step chip erase, individual overwriting, etc.) as methods to
clear or purge EEPROM. See DoD 5200.28-M for additional information.